This application claims the priority of German Application No. 19634728.9-21, filed Aug. 28, 1996, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a vehicle steering system with an electronic control system for coupling the steering wheel and steered wheels of the vehicle, in which a signal evaluation circuit receives input signals from a transducer operated by the steering wheel, and converts them with a predetermined progressivity between the change in signal on the input side on output side, to produce a steering angle setpoint signal. The system comprises a filter (or operates as such), which essentially passes only those frequencies below a transition frequency that can be varied as a function of parameters.
Currently, in conventional highway vehicles, the steering device and/or a steering wheel and the steered wheels of the vehicle are mechanically forcibly coupled together, with a servo assembly being provided as a rule to reduce the manual force required.
However, vehicle steering systems have been designed in which the steering wheel and the steered wheels of the vehicle are coupled with one another only by an electronic control system which compares a steering angle setpoint determined by the steering wheel with the corresponding actual value at the steered vehicle wheels, and adjusts the vehicle wheels accordingly. Such designs resemble electronic control systems like those known in aircraft. These systems, called "fly by wire" in aircraft, have in the meantime become so reliable that they are routinely used in passenger aircraft as well.
In steering systems with electronic control circuits for coupling the steering wheel and the steered wheels of the vehicle, it is possible to change the translation ratio between the adjusting travel of the steering wheel and the change in the steering angle of the steered wheels of the vehicle as a function of a selected group of parameters, in an especially simple manner. Such a system is disclosed, for example, in German patent document, DE 41 34 390 A1. However, a comparatively high construction cost is involved for this purpose, which in the case of the control circuit provided for coupling between the steering wheel and the steered vehicle wheels, can be replaced by appropriate computer operations for which electronics can be easily programmed with an appropriate design. As a result, therefore, it is possible at low cost in an advantageous manner, at a very low road speed, (such as that which is typical of parking maneuvers), to operate with a high translation ratio so that a small movement of the steering wheel is sufficient to change the steering angle of the steered vehicle wheels considerably.
Another advantage of steering systems with electronic control circuits for coupling the steering wheel and the steered vehicle wheels is that it is comparatively easy to perform an automatic steering correction when unanticipated disturbing forces (such as a side wind) act on the vehicle. In this connection, reference is made to DE 42 32 256 C2.
A vehicle steering system of the type mentioned above is known from German patent document DE 43 00 366 A1 and takes the form of an additional steering system for the rear wheels, with the setpoint signal generated by the regulator passing through a low pass filter.
German patent document DE 195 47 176 A1 teaches a method for determining a regulating parameter adapted to actual driving conditions in a circuit for regulating the driving stability of a vehicle, with a circuit stage that serves to generate a setpoint signal connected on the input side with lowpass filters.
The goal of the present invention is to produce a high degree of insensitivity to disturbing influences on the steering wheel in a steering system of the type recited at the outset.
This goal is achieved according to the invention by utilizing a filter that operates with a low transition frequency (that is, a low pass filter having an upper limit of the pass band which is set at a low frequency) when a direct translation exists between an adjusting movement of the steering wheel and the resulting change in the steering angle of the steered wheels of the vehicle, and operates at a high transition frequency (that is, the upper limit of the pass band is set at a higher frequency) when an indirect translation exists for the steering system between the adjusting movement of the steering wheel and the resulting change in the steering angle of the steered wheels of the vehicle.
The invention is based on the general idea that when there is an abrupt change in the input signal generated by the steering wheel and the transducer which it actuates, the output signal changes only with a presettable progressivity, but not as abruptly, in order to change the input signal to a similar degree abruptly. This can be accomplished in simple fashion by using the above mentioned filter. An abrupt change in the signal can be represented (mathematically) with theoretically any degree of accuracy as the superimposition of a great many different frequency components; in other words, an infinitely large number that can be counted with mathematical exactness as superimposition. When the filter cuts out the higher-frequency signal components above the presettable transition frequency, only signal components with relatively low frequencies will be taken into account in the superimposition. As a result, instead of an abrupt signal change, a more or less steep increase or decrease in the signal is generated.
Thus, the adjustment rate of the steered wheels of the vehicle is less than the adjustment speed of the steering wheel, so that very rapid oscillating movements of the steering device cause only comparatively minor oscillating movements of the steered wheels of the vehicle.
According to one especially preferred embodiment of the invention, the filter is set to a low transition frequency when, for example at a low driving speed, because of the correspondingly high translation ratio, a minor adjustment of the steering handle produces large changes in the steering angle of the steered wheels of the vehicle. On the other hand, for example at higher speeds, when there is a low translation ratio between the steering wheel and the steered wheels of the vehicle, the filter is set to a high transition frequency. In the latter case, movements of the steering wheel are transmitted directly and without any significant delay to the steered wheels of the vehicle, while in the former case, a delayed transmission takes place, similar to that in mechanical forced coupling with considerable elasticity.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.